Exhaust gas purification system and method for purifying exhaust gas

ABSTRACT

A previous-stage oxidation catalyst device, a Diesel Particulate Filter (DPF) device, a turbine of a turbocharger, a previous-stage NOx selective reduction catalyst device, and a subsequent-stage NOx selective reduction catalyst device in an exhaust system of an internal combustion engine in this order from an exhaust port side. An ammonia-based solution feeder is on an inlet side or outlet side of the DPF device. The previous-stage NOx selective reduction catalyst device is a rare earth composite oxide catalyst, and the subsequent-stage NOx selective reduction catalyst device is a zeolite catalyst. The NOx removal rate is improved in wide ranges from low to high temperatures and to high flow rates, and the temperature of the DPF device is kept high to increase the time and frequency of continuous regeneration, thereby reducing the frequency of forced regeneration and an amount of discharge of CO 2  during the forced regeneration.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification system andan exhaust gas purification method for removing PMs (particulatematters), NOx (nitrogen oxides), and the like in the exhaust gas of theinternal combustion engine mounted on a diesel automobile or the like.

BACKGROUND ART

In view of preservation of the global environment, automobile emissioncontrol has increasingly been advanced. In particular, diesel enginesdesigned to be mounted on vehicles have been required to decreaseparticulate matters (PMs) and nitrogen oxides (NOx). Diesel particulatefilter devices (DPF devices) have been used to decrease the PMs, andurea NOx selective reduction catalyst devices (urea SCR devices),hydrocarbon NOx selective reduction catalyst devices (HC-SCR devices),lean NOx trap catalyst device (LNT devices), and the like are used todecrease the nitrogen oxides. Removal of hazardous substances bymounting these multiple types of exhaust gas purification devices hasbeen advanced.

As one such example, an exhaust gas purification system as described inJapanese patent application Kokai publication No. 2010-242515 has beenproposed. In the system, an oxidation catalyst, a urea injection device,a diesel particulate filter device, a NOx selective reduction catalystconverter, and an oxidation catalyst are disposed in an exhaust passagein this order from an upstream side. Also, a urea decomposition catalystis supported in the diesel particulate filter device, instead ofsupporting a catalyst with an oxidizing function.

Moreover, there has also been an exhaust gas purification system 1X asillustrated in FIG. 17 including an exhaust gas purification apparatus20X in which an oxidation catalyst device 21, a diesel particulatefilter device (DPF) 22, and a NOx selective reduction catalyst device(SCR) 23X are disposed in this order from an upstream side at a positiondownstream of a turbine 14 of a turbocharger provided in an exhaustpassage 13 of an internal combustion engine 10, and a urea injectionnozzle 25 is provided between the diesel particulate filter device 22and the NOx selective reduction catalyst device 23.

With the improvement in engine combustion, fuel consumption has beenimproved and the total amount of discharge of particulate matters andnitrogen oxides has been decreased as well. On the other hand, thetemperature of exhaust gas that flows into the exhaust gas purificationapparatus has been decreased. Specifically, as a result of theimprovement in engine's combustion conditions, the temperature ofexhaust gas has been decreased by 30° C. to 50° C. or greater ascompared to conventional cases. In addition, exhaust gas purificationapparatuses have been employing multiple devices therein and thereforeincreased in size, thus increasing the thermal capacity. For thesereasons, it has become difficult to ensure the activation temperaturesof the catalysts.

In addition, in a urea SCR system, it is difficult to shorten thedistance from a urea feeder such as a urea water injection nozzle to aurea NOx selective reduction catalyst device in view of evenlydispersing urea water and accelerating decomposition of the urea toammonia. This has also been a major cause of the increase in the size ofexhaust gas purification apparatuses.

As a countermeasure to these problems, the inventor of the presentapplication has proposed a diesel engine exhaust gas purificationapparatus as described in Japanese patent application Kokai publicationNo. 2011-149400, for example. In the apparatus, previous-stage oxidationcatalysts (DOCs), a urea injection nozzle, a turbine of a turbocharger(low-pressure stage turbine), a diesel particulate filter (DPF), aselective reduction catalyst (urea SCR), and a subsequent-stageoxidation catalyst (R-DOC) are disposed in an exhaust passage in thisorder from an upstream side. With this configuration, eachpost-treatment unit is made closer to exhaust ports, so that the heat ofthe exhaust gas is effectively utilized for the each post-treatment uniteasily to ensure its catalyst activation temperature.

Meanwhile, the modes for measuring exhaust gas will switch from theconventionally used JE05 driving mode (Japanese driving mode simulatinginner-city driving), the NEDC (European driving cycle) driving mode, andthe like to a world harmonized standard, or WHDC (vehicle cycle fortesting exhaust gas emission of heavy vehicles) driving mode, and thelike. For this reason, decreasing exhaust gas during a cold mode andunder high-temperature, high-flow-rate conditions will be necessary.

On the other hand, as for urea SCR systems, controlling adsorption ofurea, its intermediate, and ammonia (NH₃) has been considered forimproving the nitrogen oxide (NOx) removal rate at low temperatures.However, there is a problem in that controlling adsorption of these isdifficult in high-temperature, high-flow-rate ranges. Moreover, as forDPF systems, the decrease in the temperature of exhaust gas passingthrough a DPF device decreases the range where continuous regenerationis possible. This increases the frequency of exhaust heating controlperformed to forcibly combust particulate matters (PMs) captured by theDPF device. Thus, a problem arises in that the amount of carbon dioxide(CO₂) discharged during forced regeneration of the DPF device increases.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese patent application Kokai publication No.2010-242515

Patent Document 2: Japanese patent application Kokai publication No.2011-149400

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in view of the above-mentionedcircumstances, and an object thereof is to provide an exhaust gaspurification system and an exhaust gas purification method capable ofimproving the NOx removal rate in wide ranges from low to hightemperatures and to high flow rates, and also keeping the temperature ofa DPF device high to increase the time and frequency of continuousregeneration, thus decreasing the frequency of forced regeneration ofthe DPF device and the amount of discharge of carbon dioxide (CO₂)produced during the forced regeneration, by using a configuration thatimproves the ammonia (NH₃) production rate and a two-stage configurationthat includes a previous-stage NOx selective reduction catalyst devicefor high temperatures and a subsequent-stage NOx selective reductioncatalyst device for low temperatures.

Means for Solving the Problems

An exhaust gas purification system of the present invention forachieving the above-mentioned object is an exhaust gas purificationsystem for removing particulate matters and nitrogen oxides in anexhaust gas of an internal combustion engine, in which a previous-stageoxidation catalyst device, a diesel particulate filter device, a turbineof a turbocharger, a previous-stage NOx selective reduction catalystdevice, and a subsequent-stage NOx selective reduction catalyst deviceare disposed in an exhaust system of the internal combustion engine inthis order from an exhaust port side, an ammonia-based solution feederis disposed between the previous-stage oxidation catalyst device and thediesel particulate filter device or between the diesel particulatefilter device and the turbine, and a NOx selective reduction catalyst(SCR catalyst) of the previous-stage NOx selective reduction catalystdevice is made of a catalyst containing a rare earth composite oxide(such as a Ce—Zr—O-based composite oxide), and a NOx selective reductioncatalyst (SCR catalyst) of the subsequent-stage NOx selective reductioncatalyst device is made of a zeolite catalyst.

According to this configuration, the ammonia-based solution feeder suchas a urea injection nozzle for feeding an ammonia-based solutioncontaining urea or the like is disposed upstream of the turbine. Thus,the position of the ammonia-based solution feeder can be close to theinternal combustion engine, and the temperature of the exhaust gas to befed with the ammonia-based solution can be kept high as compared to thearrangements of conventional techniques. Accordingly, the rate ofproduction of NH₃ (ammonia) from the ammonia-based solution can beimproved.

Moreover, there are disposed the previous-stage NOx selective reductioncatalyst device for high temperatures made of a catalyst containing arare earth composite oxide such as a Ce—Zr—O-based composite oxide, andthe subsequent-stage NOx selective reduction catalyst device for lowtemperatures made of a zeolite catalyst. With this two-stageconfiguration, it is possible to improve the NOx removal rate in wideranges from low to high temperatures and to high flow rates.

Moreover, since the diesel particulate filter device (DPF device) isdisposed upstream of the turbine, the position of the DPF device isclose to the exhaust port. In this way, the temperature of the exhaustgas at the inlet of the DPF device can be kept higher by 100° C. orgreater than those of the arrangements of the conventional techniques.This makes it possible to increase the time and frequency of continuousregeneration of the DPF device. As a result, the size of the DPF devicecan be decreased and the heating time during regeneration can beshortened, thereby making it possible to decrease the amount ofdischarge of CO₂ during the regeneration of the DPF device. In additionto this, the degree of freedom in layout can be increased.

Further, in the case of employing the configuration in which theammonia-based solution feeder is disposed between the previous-stageoxidation catalyst device and the DPF device, the ammonia-based solutionfeeder is disposed upstream of the DPF device and can therefore becloser to the internal combustion engine. In this way, the temperatureof the exhaust gas to be fed with the ammonia-based solution can be kepthigher by 100° C. or greater than those of the arrangements of theconventional techniques. Accordingly, the NH₃ production rate canfurther be improved.

Moreover, in the case of employing the configuration in which theammonia-based solution feeder is disposed between the previous-stageoxidation catalyst device and the diesel particulate filter device, theammonia-based solution feeder, the DPF device, and the turbine aredisposed in this order. In this way, SOx (sulfur oxides) produced bycombustion inside the cylinder is changed to CaSO₄ (calcium sulfate),which is has low corrosive properties, through a chemical reaction withNH₃ (ammonia) produced from the ammonia-based solution containing ureaor the like fed from the ammonia-based solution feeder such as a ureainjection nozzle, and with an ash component produced after thecombustion of PMs in the DPF device. Thus, it is possible to suppresscorrosion of the turbine by SOx produced by high-pressure EGRcombustion. Further, since the DPF device is disposed in such a way asnot to be influenced by ash originating from the oil of the turbine, itis possible to avoid the influence of the ash on clogging of the DPFdevice.

In addition, an EGR passage can be provided upstream of the turbine butimmediately downstream of the oxidation catalyst device or immediatelydownstream of the DPF device to take out an EGR gas. Thus, the EGRpassage can be shortened. Further, since the EGR gas is the exhaust gasfrom which HC and PMs have been removed, the above configuration iseffective as a countermeasure to prevent contamination in the EGRpassage.

The above-described exhaust gas purification system may compriseammonia-based solution feed controlling means for finding, from anequivalence ratio of a chemical reaction, an amount which enablesreduction of an amount of NOx discharged from the internal combustionengine, calculating a first ammonia-based solution amount larger thanthe amount enabling the reduction, calculating a second ammonia-basedsolution amount from a difference between a NOx target discharge amountfrom the internal combustion engine, and an amount of NOx measureddownstream of the subsequent-stage NOx selective reduction catalystdevice, setting an amount of an ammonia-based solution to be fed to theexhaust system based on the sum of the first ammonia-based solutionamount and the second ammonia-based solution amount, and feeding theammonia-based solution from the ammonia-based solution feeder. With thisconfiguration, the amount of feed of the ammonia-based solution can be amore appropriate amount. Thus, NOx can be removed efficiently.

Specifically, in the case of the arrangements of the conventionaltechniques, urea is injected upstream of the NOx selective reductioncatalyst (SCR catalyst), and urea, an intermediate of urea, NH₃, and thelike (urea-derived substances) and NOx are adsorbed to the catalystsurface of the NOx selective reduction catalyst device (SCR device). Theamount of injection of urea is controlled based on the amount of theengine-out NOx (a NH₃/NO equivalence ratio of 1 or greater). Suchcontrol works effectively when the temperature at the inlet of the NOxselective reduction catalyst device is low (300° C. or lower). However,the urea-derived substances and NOx after the adsorption desorb as soonas the temperature becomes high (above 300° C.) Thus, there is a problemin that a high NOx removal rate cannot be achieved with adsorptioncontrol performed similarly to when the temperature is low.

On the other hand, in the present invention, the previous-stage NOxselective reduction catalyst device is used for NOx removal on ahigh-temperature side, and the subsequent-stage NOx selective reductioncatalyst device is used for NOx removal on a low-temperature side. Forthe NOx removal by the NOx selective reduction catalyst devices at twostages, the first ammonia-based solution amount is calculated for theupstream, previous-stage NOx selective reduction catalyst device, thefirst ammonia-based solution amount being within a range in which theamount of urea relative to the amount of the engine-out NOx is equal toan NH₃/NO equivalence ratio of 1.0 to 1.3. Also, the secondammonia-based solution amount, which is equal to the amount of shortageof the ammonia-based solution, is calculated for the downstream,subsequent-stage NOx selective reduction catalyst device from the amountof discharge of NOx downstream of the subsequent-stage NOx selectivereduction catalyst device and added to the first ammonia-based solutionamount, by which the ammonia-based solution is then fed.

The above-described exhaust gas purification system may comprisehydrocarbon feed controlling means for performing control in which ahydrocarbon is fed into the exhaust gas upstream of the previous-stageoxidation catalyst device by post injection via injection inside acylinder or by exhaust pipe fuel injection in a case where adifferential pressure between upstream and downstream sides of thediesel particulate filter device is equal to or higher than a continuousregeneration determination differential pressure but equal to or lowerthan an automatic forced regeneration determination differentialpressure, and a temperature of the exhaust gas at an inlet of the dieselparticulate filter device is equal to or lower than a continuousregeneration control start temperature. With this configuration, by theeffect of HC adsorption and oxidation of the oxidation catalyst in theprevious-stage oxidation catalyst device upstream of the DPF device,when continuous regeneration of the DPF device is needed, thetemperature of the exhaust gas that flows into the DPF device (thetemperature of the exhaust gas at the inlet) can be raised to atemperature at which continuous regeneration is possible. In this way,the interval of the automatic forced regeneration control for the DPFdevice can be extended. Accordingly, the amount of discharge of CO₂during the regeneration of the DPF device can further be decreased.

An exhaust gas purification method of the present invention forachieving the above-mentioned object is an exhaust gas purificationmethod for removing particulate matters and nitrogen oxides in anexhaust gas of an internal combustion engine by using an exhaust gaspurification system in which a previous-stage oxidation catalyst device,a diesel particulate filter device, a turbine of a turbocharger, aprevious-stage NOx selective reduction catalyst device, and asubsequent-stage NOx selective reduction catalyst device are disposed inan exhaust system of the internal combustion engine in this order froman exhaust port side, and an ammonia-based solution feeder is disposedbetween the previous-stage oxidation catalyst device and the dieselparticulate filter device or between the diesel particulate filterdevice and the turbine, characterized in that the method comprises:finding, from an equivalence ratio of a chemical equation, an amountwhich enables reduction of an amount of NOx discharged from the internalcombustion engine; calculating a first ammonia-based solution amountlarger than the amount enabling the reduction; calculating a secondammonia-based solution amount from a difference between a NOx targetdischarge amount from the internal combustion engine, and an amount ofNOx measured downstream of the subsequent-stage NOx selective reductioncatalyst device; setting an amount of an ammonia-based solution to befed to the exhaust system based on the sum of the first ammonia-basedsolution amount and the second ammonia-based solution amount; andfeeding the ammonia-based solution from the ammonia-based solutionfeeder.

With this method, the amount of feed of the ammonia-based solution canbe a more appropriate amount. Thus, NOx can be removed efficiently.

The above-described exhaust gas purification method may further comprisefeeding a hydrocarbon into the exhaust gas upstream of theprevious-stage oxidation catalyst device by post injection via injectioninside a cylinder or by exhaust pipe fuel injection in a case where adifferential pressure between upstream and downstream sides of thediesel particulate filter device is equal to or higher than a continuousregeneration determination differential pressure but equal to or lowerthan an automatic forced regeneration determination differentialpressure, and a temperature of the exhaust gas at an inlet of the dieselparticulate filter device is equal to or lower than a continuousregeneration control start temperature. In this way, by the effect of HCadsorption and oxidation of the oxidation catalyst in the previous-stageoxidation catalyst device upstream of the DPF device, when continuousregeneration of the DPF device is needed, the temperature of the exhaustgas that flows into the DPF device (the temperature of the exhaust gasat the inlet) can be raised to a temperature at which continuousregeneration is possible. In this way, the interval of the automaticforced regeneration control for the DPF device can be extended.Accordingly, the amount of discharge of CO₂ during the regeneration ofthe DPF device can further be decreased.

Effects of the Invention

With the exhaust gas purification system and the exhaust gaspurification according to the present invention, the urea injectionnozzle can be disposed significantly close to the engine body. Thus, itis possible to improve the ammonia (NH₃) production rate and thereforeimprove the NOx removal rate. In addition to this, with the two-stageconfiguration including the previous-stage NOx selective reductioncatalyst device for high temperatures and the subsequent-stage NOxselective reduction catalyst device for low temperatures, it is possibleto improve the NOx removal rate in wide ranges from low to hightemperatures and to high flow rates.

Moreover, since the DPF device is disposed upstream of the turbine, theposition of the DPF device is close to the exhaust port, and thereforethe temperature of the DPF device can be kept high. This makes itpossible to increase the time and frequency of continuous regenerationand decrease the size. The decrease in the size of the DPF can shortenthe heating time during regeneration. Thus, it is possible to decreasethe amount of discharge of CO₂ during regeneration of the DPF device andalso to increase the degree of freedom in layout.

Further, since the DPF device is disposed upstream of the turbine and isnot influenced by ash originating from the oil of the turbine, it ispossible to avoid the influence of the ash on clogging of the DPFdevice.

In addition, in the case where the ammonia-based solution feeder isdisposed between the previous-stage oxidation catalyst device and theDPF device, the ammonia-based solution feeder, the DPF device, and theturbine are disposed in this order. In this way, sulfur oxides (SOx)produced by combustion in the cylinder can be changed to calcium sulfate(CASO₄), which has low corrosive properties, through a reaction withcalcium carbonate (CaCO₃) produced by the combustion of particulatematters (PMs) captured by the DPF device. Thus, it is possible tosuppress corrosion of the turbine of the turbocharger disposeddownstream of the DPF device by the sulfur components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of an exhaust gas purification system of an embodimentof the present invention, illustrating a configuration in whichprevious-stage oxidation catalyst devices are provided at a singlestage.

FIG. 2 is a view of the exhaust gas purification system in FIG. 1,illustrating a configuration in which the position of a urea injectionnozzle is different.

FIG. 3 is a view of an exhaust gas purification system of an embodimentof the present invention, illustrating a configuration in whichprevious-stage oxidation catalyst devices are provided at two stages.

FIG. 4 is a view of the exhaust gas purification system in FIG. 3,illustrating a configuration in which the position of a urea injectionnozzle is different.

FIG. 5 is a chart illustrating one exemplary control flow of hydrocarbonfeed control of the present invention.

FIG. 6 is one exemplary control flow of urea feed control of the presentinvention.

FIG. 7 is a graph illustrating the relationship between the proportionof decrease in the diameter of a DPF device and DPF pressure loss inExample and Conventional Example.

FIG. 8 is a graph illustrating the relationship between the proportionof decrease in the diameter of the DPF device and exhaust manifoldpressure in Example and Conventional Example.

FIG. 9 is a graph illustrating the relationship between the proportionof decrease in the diameter of the DPF device and engine torque inExample and Conventional Example.

FIG. 10 is a graph illustrating the time for which the DPF device isheated in Example and Conventional Example.

FIG. 11 is a graph illustrating DPF inlet temperature in the JE05 modein Example and Conventional Example.

FIG. 12 is a graph illustrating the relationship between SCR deviceinlet temperature and the rate of production of NH₃ from urea in Exampleand Conventional Example.

FIG. 13 is a graph illustrating the relationship between turbine inlettemperature and the NOx removal rate in Example and ConventionalExample.

FIG. 14 is a graph illustrating an average DPF regeneration interval inExample and Conventional Example.

FIG. 15 is a graph illustrating the ratio of the amount of discharge ofCO₂ between Example and Conventional Example.

FIG. 16 is a graph illustrating an average NOx removal rate in Exampleand Conventional Example.

FIG. 17 is a view illustrating an exemplary configuration of an exhaustgas purification system of a conventional technique.

MODES FOR CARRYING OUT THE INVENTION

Hereinbelow, exhaust gas purification systems and exhaust gaspurification methods of embodiments according to the present inventionwill be described with reference to the drawings. Here, an example willbe given in which a NOx selective reduction catalyst is a urea NOxselective reduction catalyst, and an ammonia-based solution is urea.However, the present invention is not limited to this example and mayemploy an HC-NOx selective reduction catalyst or the like.

As illustrated in FIG. 1, an exhaust gas purification system 1 of anembodiment according to the present invention is an exhaust gaspurification system for removing PMs (particulate matters) and NOx(nitrogen oxides) in an exhaust gas G of an internal combustion engine(hereinafter referred to as the engine) 10 such as a diesel engine. Theexhaust gas purification system 1 is configured such that aprevious-stage oxidation catalyst device (DOC) 21, a diesel particulatefilter device (hereinafter referred to as the DPF device) 22, a turbine14 of a turbocharger, a previous-stage NOx selective reduction catalystdevice (hereinafter referred as the previous-stage SCR device) 23, and asubsequent-stage NOx selective reduction catalyst device (hereinafterreferred to as the subsequent-stage SCR device) 24 are disposed in theexhaust system of the engine 10 in this order from an exhaust port sideconnected to an engine body 11.

Moreover, a urea injection nozzle 25 as an ammonia-based solution feederis disposed between the previous-stage oxidation catalyst device (DOC)21 and the DPF device 22 as illustrated in FIG. 1. The urea injectionnozzle 25 may be disposed between the DPF device 22 and the turbine 14as illustrated in FIG. 2.

As illustrated in FIGS. 1 and 2, the previous-stage oxidation catalystdevice 21 is disposed for each exhaust port in an exhaust manifold 12 sothat each previous-stage oxidation catalyst device 21 can contact theexhaust gas G as long as possible and thereby increase the time forwhich the supported oxidation catalyst is at or above its activationtemperature. Moreover, if necessary, a subsequent-stage oxidationcatalyst device (R-DOC: not illustrated) is disposed downstream of thesubsequent-stage SCR device 24 for NH₃ (ammonia) slip, so as todecompose NH₃ flowing out of the subsequent-stage SCR device 24.

Each previous-stage oxidation catalyst device 21 is formed by disposingcatalyst layers containing a metal catalyst which is excellent inremoval of CO (carbon monoxide) and a catalyst in which an oxide with anoxygen storage capacity (OSC) and an oxide semiconductor exist in amixed state. As the oxide with an oxygen storage capacity, an oxidecontaining Ce (cerium) is available. As the oxide semiconductor, TiO₂(titanium dioxide), ZnO (zinc oxide), Y₂O₃ (yttrium oxide), and the likeare available. Moreover, a precious metal is supported on the oxide withan oxygen storage capacity.

Meanwhile, depending on the exhaust temperature, HC (hydrocarbon)concentration, and CO concentration, the exhaust gas purification system1 may employ a configuration in which the previous-stage oxidationcatalyst devices 21 are provided at a single stage as illustrated inFIGS. 1 and 2. However, in a case where the HC concentration and COconcentration in the exhaust gas are high, the previous-stage oxidationcatalyst devices 21 are preferably divided and disposed as a firstoxidation catalyst device (DOC-1) 21 a and a second oxidation catalystdevice (DOC-2) 21 b so that a catalyst configuration excellent inlow-temperature activation can be obtained. In this case, like anexhaust gas purification systems 1A illustrated in FIGS. 3 and 4, thefirst oxidation catalyst device 21 a is disposed for each cylinder andexhaust port in the exhaust manifold 12, while the second oxidationcatalyst device 21 b is disposed downstream of the outlet of the exhaustmanifold 12. Note that the configurations in FIGS. 3 and 4 are the same,except that the urea injection nozzle 25 as the ammonia-based solutionfeeder is disposed between the previous-stage oxidation catalyst devices(DOC) 21 and the DPF device 22 in FIG. 3, whereas the urea injectionnozzle 25 is disposed between the DPF device 22 and the turbine 14 inFIG. 4.

In each first oxidation catalyst device 21 a, catalyst layers aredisposed which contain a metal catalyst which is excellent in COremoval, a catalyst in which an oxide with an OSC such as an oxidecontaining cerium (Ce), and an oxide semiconductor such as TiO₂, ZnO, orY₂O₃ exist in a mixed state. Moreover, a precious metal is supported onthe oxide with an OSC. On the other hand, in the second oxidationcatalyst device 21 b, catalyst layers are disposed which contain acatalyst of a precious metal such as platinum (Pt) excellent in HCremoval, or a catalyst in which a HC adsorbing material and a preciousmetal catalyst exist in a mixed state. With these, a catalystconfiguration excellent in low-temperature activation can be obtained.

For the DPF device 22, it is preferable to use a small DPF decreased involume by 50% or more as compared to those by conventional techniques,by forming a structure with optimized porosity, pore diameter, and wallthickness that allow equivalent purification characteristics and also asmaller pressure loss. Further, in a case where urea L is to be sprayedupstream of the DPF device 22 (in the case of the configurations inFIGS. 1 and 3), the DPF device 22 is coated with no precious metalcatalyst so as to prevent oxidation of the urea L. In this case, a DPFcoated with no catalyst, or a DPF coated with a highly basic rare earthoxide- or alkaline earth oxide-based catalyst is used. Moreover, bycoating the DPF with a hydrolysis catalyst, the NH₃ production rate canfurther be improved, and the NOx removal rate can therefore be improved.

In the previous-stage SCR device 23, a catalyst with high NOx removalperformance at high temperatures is preferably supported, such as a NOxselective reduction catalyst (hereinafter referred to as the SCRcatalyst) made of a catalyst containing a rare earth composite oxide(such as a Ce—Zr—O-based composite oxide). On the other hand, in thesubsequent-stage SCR device 24, a zeolite catalyst having a function ofadsorbing urea-derived substances and NOx at low temperatures ispreferably supported. It is also preferable to use a small SCR deviceusing a characteristic catalyst support (monolithic catalyst) or thelike to increase the amount of the catalyst per specific volume so thata 50% or more decrease in size can be achieved as compared toconventional cases.

Since the urea injection nozzle 25 is disposed upstream of the turbine14, the urea L is injected upstream of the turbine 14. Thus, the urea Linjected into the exhaust gas G is agitated and dispersed inside theturbine 14. Accordingly, the hydrolysis and pyrolysis of the urea L areaccelerated. Further, the dispersion of the spray in an exhaust passage13 after passing through the turbine 14 becomes uniform. Thus, thedistance from the urea injection nozzle 25 to the previous-stage SCRdevice 23 can be shortened, and closer arrangement can therefore beobtained.

Moreover, an HP-EGR passage 15 and a LP-EGR passage 16 for performingEGR to decrease NOx are provided. The HP-EGR passage 15 separates an EGRgas Ge to be recirculated for HP (high pressure)-EGR after it passes theprevious-stage oxidation catalyst devices 21 (or the first oxidationcatalyst devices 21 a in FIG. 3 or 4) at the exhaust passage 13 upstreamof the urea injection nozzle 25. In this way, the EGR gas Ge afterpassing through the previous-stage oxidation catalyst devices 21 isrecirculated to the HP-EGR passage 15. Thus, the SOF (soluble organicfraction) in the EGR gas Ge in the HP-EGR passage 15 can be decreased.Accordingly, it is possible to decrease the influence of the SOF such asclogging of an EGR cooler (not illustrated) and an EGR valve (notillustrated) of the HP-EGR passage 15.

Further, the EGR gas Ge to be recirculated for LP (low-pressure)-EGR isseparated downstream of the previous-stage SCR device 23 or thesubsequent-stage SCR device 24 (the subsequent-stage SCR device 24 inFIGS. 1 to 4). In this way, the EGR gas Ge after passing through theprevious-stage oxidation catalyst devices 21 (or the first oxidationcatalyst devices 21 a and the second oxidation catalyst device 21 b inFIGS. 3 and 4), the DPF device 22, the previous-stage SCR device 23, andthe subsequent-stage SCR device 24 is recirculated to the LP-EGR passage16. Thus, the SOF, PMs, and NH₃ in the EGR gas Ge in the LP-EGR passage16 can be decreased. Accordingly, it is possible to decrease closing,corrosion, and the like of an EGR cooler (not illustrated) and an EGRvalve (not illustrated) of the LP-EGR passage 16.

The exhaust gas purification systems 1, 1A further include a temperaturesensor 31 which measures a DPF inlet temperature T, which is thetemperature of the exhaust gas at the inlet of the DPF device 22, adifferential pressure sensor 32 which measures a differential pressureΔP between upstream and downstream sides of the DPF device 22, and a NOxconcentration sensor 33 which measures the concentration of NOxdownstream of the subsequent-stage SCR device 24. The exhaust gaspurification systems 1, 1A further include a control device (notillustrated) including HC feed controlling means for inputting themeasurement values of the temperature sensor 31 and differentialpressure sensor 32 and feeding HC (hydrocarbons) as a fuel into theprevious-stage oxidation catalyst devices 21 by post-injection insidethe cylinders, and urea feed controlling means (ammonia-based solutionfeed controlling means) for feeding the urea L for producing NH₃ for NOxreduction in the previous-stage SCR device 23 and the subsequent-stageSCR device 24, from the urea injection nozzle 25 into the exhaust gas G.Usually, a control device (not illustrated) called an ECU (enginecontrol unit) which controls the entire operation of the engine 10functions also as the above control device. In other words, the HC feedcontrolling means and the urea feed controlling means are incorporatedin the control device (ECU).

FIGS. 7 to 9 illustrate DPF pressure loss and the like in Example A inwhich the DPF device 22 is disposed upstream of the turbine 14 and inConventional Example B in which the DPF device 22 is disposed downstreamof the turbine. In Example A, the DPF pressure loss is decreased, thepressure inside the exhaust manifold 12 is decreased, and the torque isincreased, as compared to Conventional Example B. In other words, sincethere is no influence of the turbine expansion ratio, the influence ofincrease in DPF pressure loss on the exhaust manifold internal pressureand the torque is relatively smaller in Example A than in ConventionalExample B.

As can be seen from these FIGS. 7 to 9, provided that the torque, theexhaust manifold internal pressure, and the like that influence theengine performance are substantially the same and that the length of theDPF device 22 is the same, the diameter of the DPF device 22 can bedecreased by about 40% in Example A as compared to Conventional ExampleB. As a result, as illustrated in FIG. 10, the DPF device 22 can beheated within a shorter time in Example A than in Conventional ExampleB, and therefore the time for heating to a predetermined temperature canbe shortened.

Moreover, in the present invention, since the DPF device 22 ispositioned upstream of the turbine 14, the DPF device 22 can be disposedcloser to the engine body 11 than in the conventional technique. As aresult, as illustrated in FIG. 11, the DPF inlet temperature T can bekept higher by 100° C. or more.

Further, since the urea injection nozzle 25 can also be disposed closerto the engine body 11 than in the conventional technique, thetemperature at the urea injection position can be kept higher than inConventional Example B. In particular, in the case of the configurationin which the urea injection nozzle 25 is disposed upstream of the DPFdevice 22 as illustrated in FIGS. 1 and 3, the temperature at the ureainjection position can be kept higher by 100° C. or more than inConventional Example B, like the DPF inlet temperature T illustrated inFIG. 11. As a result, as illustrated in FIG. 12, the rate of productionof NH₃ from urea with respect to the temperature at the inlet of theprevious-stage SCR device 23 improves significantly in Example A ascompared to Conventional Example B. Thus, as illustrated in FIG. 13, theNOx removal rate with respect to the turbine outlet temperature improvesas well.

Next, HC (hydrocarbon) feed control in the above-described exhaust gaspurification systems 1, 1A will be described. In the present invention,in view of the superiority of the above-described configuration, HC feedto each previous-stage oxidation catalyst device 21 is controlled suchthat HC adsorption and oxidation in the previous-stage oxidationcatalyst device 21 raise the temperature of the exhaust gas G that flowsinto the DPF device 22, to thereby set the DPF inlet temperature T,which is the temperature of the exhaust gas at the inlet of the DPFdevice 22, to a temperature (250° to 500° C.) that allows continuousregeneration. In this way, the frequency and duration at and for whichcontinuous regeneration can be performed are increased.

This HC feed control can be performed through a control flow exemplarilyillustrated in FIG. 5. The control flow in FIG. 5 is illustrated as acontrol flow which is repeatedly called and executed by a higher controlflow which is actuated upon start of operation of the engine 10; thecontrol flow is discontinued and returns to the higher control flow uponstop of operation of the engine 10, and stops as the higher control flowstops.

When the control flow in FIG. 5 is called by the higher control flow andstarts, in step S11, the DPF inlet temperature T is inputted from thetemperature sensor 31, and the DPF upstream-downstream differentialpressure ΔP, which is the differential pressure between the upstream anddownstream sides of the DPF device 22, is inputted from the differentialpressure sensor 32. In the next step S12, it is determined whether ornot the DPF upstream-downstream differential pressure ΔP is equal to orhigher than a continuous regeneration determination differentialpressure ΔPL. If the DPF upstream-downstream differential pressure ΔP isequal to or higher than the continuous regeneration determinationdifferential pressure ΔPL (YES), it is determined in the next step S13whether or not the DPF upstream-downstream differential pressure ΔP isequal to or lower than an automatic forced regeneration determinationdifferential pressure ΔPH. If the DPF upstream-downstream differentialpressure ΔP is equal to or lower than the automatic forced regenerationdetermination differential pressure ΔPH (YES), the flow proceeds to stepS14.

Note that if it is determined in step S12 that the DPFupstream-downstream differential pressure ΔP is lower than thecontinuous regeneration determination differential pressure ΔPL (NO),the flow returns to step S11. Moreover, if it is determined in step S13that the DPF upstream-downstream differential pressure ΔP is higher thanthe automatic forced regeneration determination differential pressureΔPH (NO), the flow proceeds to step S20, in which automatic forcedregeneration control is performed to forcibly regenerate the DPF device22. Then, the flow returns to the higher control flow, and the controlflow in FIG. 5 is called again by the higher control flow and isrepeated.

In step S14, it is determined whether or not the DPF inlet temperature Tis equal to or lower than a continuous regeneration control starttemperature TL. If the DPF inlet temperature T is equal to or lower thanthe continuous regeneration control start temperature TL (YES), HC feedis performed in step S15 in which HC is fed to the previous-stageoxidation catalyst devices 21 by post-injection for a predetermined timeΔt1 (a time set in advance according to the intervals of thedetermination of the DPF upstream-downstream differential pressure ΔPand the determination of the DPF inlet temperature T). Thereafter, theflow returns to step S14. If the DPF inlet temperature T is higher thanthe continuous regeneration control start temperature TL (NO) in stepS14, the flow proceeds to step S16.

In step S16, since the DPF inlet temperature T is higher than thecontinuous regeneration control start temperature TL, the flow waits fora predetermined time Δt2, during which continuous regeneration of theDPF device 22 is performed. Thereafter, the flow proceeds to step S17,in which, if HC feed is being performed, the HC feed is stopped, and ifno HC feed is being performed, HC feed is kept stopped, and the flowproceeds to step S18.

In step S18, it is determined whether or not the DPF upstream-downstreamdifferential pressure ΔP is equal to or lower than the continuousregeneration determination differential pressure ΔPL. If the DPFupstream-downstream differential pressure ΔP is neither equal to norlower than the continuous regeneration determination differentialpressure ΔPL (NO), the flow returns to step S14 to continue thecontinuous regeneration. If the DPF upstream-downstream differentialpressure ΔP is equal to or lower than the continuous regenerationdetermination differential pressure ΔPL (YES), the continuousregeneration is considered to have been completed and to be unrequired.Thus, the flow returns to the higher control flow, and the control flowin FIG. 5 is called again by the higher control flow, and starts and isrepeated again.

It is determined in steps S11 to S13 whether or not to feed HC forheating the exhaust gas for continuous regeneration and steps S14, S15are repeated, to thereby raise the DPF inlet temperature T until itexceeds the continuous regeneration control start temperature TL. Then,continuous regeneration is performed in step S16, and the HC feed isstopped in step S17 to prevent unnecessary HC consumption. Thereafter,it is determined in step S18 whether or not to end the continuousregeneration.

By performing the control flow in FIG. 5, it is possible to performcontrol in which HC is feed into the exhaust gas G upstream of theprevious-stage oxidation catalyst devices 21 by post-injection viainjection inside the cylinders in the case where the DPFupstream-downstream differential pressure ΔP of the DPF device 22 isequal to or higher than the continuous regeneration determinationdifferential pressure ΔPL but equal to or lower than the automaticforced regeneration determination differential pressure ΔPH and also theDPF inlet temperature T of the DPF device 22 is equal to or lower thanthe continuous regeneration control start temperature TL. Note thatinstead of the post-injection, exhaust pipe fuel injection may beemployed in which fuel is injected directly into the exhaust pipesupstream of the previous-stage oxidation catalyst devices 21.

As illustrated in FIG. 14, by this HC feed control, the interval ofautomatic forced regeneration of the DPF device 22 can be greatlyextended in Example A of the present invention as compared toConventional Example B of the conventional technique. Further, asillustrated in FIG. 15, the amount of discharge of CO₂ during DPFregeneration can be significantly decreased in Example A as compared toConventional Example B. Note that a material capable of adsorption of alarge amount of CO, such as CeO₂ (cerium oxide) or ZrO₂ (zirconiumdioxide), may be used for the oxidation catalyst of each previous-stageoxidation catalyst device 21. In this way, the amount of heat productionof the previous-stage oxidation catalyst device 21 can further beincreased.

By this HC feed control, the effect of HC adsorption and oxidation ofthe oxidation catalyst in each previous-stage oxidation catalyst device21 upstream of the DPF device 22 can be exhibited effectively. Thus,when continuous regeneration of the DPF device 22 is needed, thetemperature T of the exhaust gas that flows into the DPF device 22(temperature of the exhaust gas at the inlet) can be raised above thetemperature TL above which continuous regeneration is possible. In thisway, the interval of automatic forced regeneration control for the DPFdevice 22 can be extended. Accordingly, the amount of discharge of CO₂during regeneration of the DPF device 22 can further be decreased.

Next, urea feed control in the above-described exhaust gas purificationsystems 1, 1A will be described. In the present invention, in view ofthe superiority of the above-described configuration, the urea feed fromthe urea injection nozzle 25 to the previous-stage SCR device 23 and thesubsequent-stage SCR device 24 is controlled such that NOx in theexhaust gas is reduced in the previous-stage SCR device 23 and thesubsequent-stage SCR device 24 with NH₃ produced from the urea L.

This urea feed control can be performed through a control flowexemplarily illustrated in FIG. 6. The control flow in FIG. 6 isillustrated as a control flow which is called and executed by a highercontrol flow that is actuated upon start of operation of the engine 10;the control flow is discontinued by an interruption of step S40 uponstop of operation of the engine 10, returns to the higher control flow,and stops as the higher control flow stops.

When the control flow in FIG. 6 is called by the higher control flow andstarts, in step S31, a first NOx discharge amount Win is measured orcalculated. The first NOx discharge amount Win is the amount ofdischarge of NOx representing the NOx (NO, NO₂) from the engine body 11converted into NO (the amount of discharge of the engine-out NOx). It isfound from a measured concentration of NOx in the exhaust gas G and acalculated amount of exhaust gas, or calculated based, for example, on acalculation involving referring to preset map data based on theoperating state of the engine 10.

Further, in step S31, a first urea feed amount Wumol for the first NOxdischarge amount Win is calculated. The first urea feed amount Wumol isobtained by calculating the amount of NH₃ for the first NOx dischargeamount that is necessary for NO reduction at an equivalence ratio of NH₃to NO of 1.0 to 1.3 (a value found and set in advance through a test orthe like), and setting the amount of urea for producing this amount ofNH₃ as the first urea feed amount Wumol. Then, the counting of a ureafeed elapsed time t is started. Also, a second urea feed amount Wuplasto be used later is set to zero.

Then, in step 32, it is determined whether or not the urea feed elapsedtime t that is being counted has reached a preset determination time t1.The determination time t1 is set to a time long enough for the exhaustgas G to reach the NOx concentration sensor 33 downstream of thesubsequent-stage SCR device 24, the exhaust gas G containing the urea Lwhich has been fed thereinto upstream of the DPF device 22 from the ureainjection nozzle 25. This time can be set based on an experimental valueor a value calculated from the flow rate of the exhaust gas or the like.

If the urea feed elapsed time t has reached the determination time t1(YES) in step S32, the flow proceeds to step S33. Alternatively, if theurea feed elapsed time t has not reached the determination time t1 (NO),the flow proceeds to step S34, in which the urea L of the first ureafeed amount Wumol is fed into the exhaust gas G upstream of the turbine14 (further upstream of the DPF device 22 in FIGS. 1 and 3) from theurea injection nozzle 25 for a preset time (a time based on the timeinterval of the determination in step S32) Δt1. Thereafter, the flowreturns to step S31.

In step S33, the measurement value of the NOx concentration sensor 33downstream of the subsequent-stage SCR device 24 is inputted, and ameasured discharge amount Wout is calculated from the NOx concentrationthus inputted and the amount of exhaust gas. Note that the amount ofexhaust gas can be calculated from the operating state of the engine 10or the amount of intake measured with an intake sensor (MAF sensor: notillustrated) and the amount of fuel injection.

The measured discharge amount Wout and a target discharge amount WT,which is a target value for decreasing NOx discharge, are compared. Ifthe measured discharge amount Wout is equal to or smaller than thetarget discharge amount WT (YES), the first urea feed amount Wumol isdetermined as a sufficient amount of urea, and the flow proceeds to stepS34, in which the urea L of the first urea feed amount Wumol is fed forthe preset time Δt1. Thereafter, the flow returns to step S31.

On the other hand, if it is determined in step S33 that the measureddischarge amount Wout is larger than the target discharge amount WT(NO), the first urea feed amount Wumol is determined as an insufficientamount of urea, and the flow proceeds to step S35.

In step S35, the measured discharge amount Wout is newly calculated, andan discharge amount difference Wdef being the difference between thetarget discharge amount WT and the measured discharge amount Wout iscalculated (Wdef=WT−Wout). Moreover, the amount of NH₃ is calculated forthe discharge amount difference Wdef which is necessary for reduction ofNOx of an amount equal to the discharge amount difference Wdef, and thesecond urea feed amount Wuplas is calculated by using a urea amount Wudfor producing this amount of NH₃; in other words, Wuplas=Wuplas+Wud.Thus, the second urea feed amount Wuplas taking the discharge amountdifference Wdef into account can be calculated. Furthermore, a totalurea feed amount Wut being the sum of the first urea feed amount Wumoland the second urea feed amount Wuplas is calculated (Wut=Wumol+Wuplas).

In the next step S36, the total urea feed amount Wut of urea L is fedfor a preset time (a time based on the time interval of update of themeasured value of NOx concentration in step S35) Δt2. The flow thenreturns to step S35. These steps S35 to S36 are repeated to feed theurea L of the total urea feed amount Wut into the exhaust gas G upstreamof the turbine 14 (further upstream of the DPF device 22 in FIGS. 1 and3). When an interruption of step S40 occurs upon stop of the engine 10,the flow returns to the higher control flow, and the control flow inFIG. 6 ends along with the higher control flow.

According to the control described above, the first urea feed amountWumol of urea L can be fed in steps S31 to S34 in the case where theurea feed elapsed time t has not reached the predetermined determinationtime t1 (NO), or the measured discharge amount Wout is equal to orsmaller than the target discharge amount WT (YES); on the other hand,the urea L of the total urea feed amount Wut, which is the sum of thefirst urea feed amount Wumol and the second urea feed amount Wuplas, canbe fed in steps S35 and S36 in the case where the urea feed elapsed timet has reached the predetermined determination time t1 (YES), and themeasured discharge amount Wout is larger than the target dischargeamount WT (NO).

In other words, urea injection control can be performed which includes:considering an amount of feed of urea, which is the amount of ureaexpected to be consumed by the previous-stage SCR device 23, as thefirst urea feed amount Wumol which allows an equivalence ratio of theamount of ammonia (NH₃) as urea to the amount of the engine-out NOx of 1or greater; further, estimating the measured discharge amount Wout ofNOx downstream of the subsequent-stage SCR device 24 from the measuredNOx concentration; calculating the discharge amount difference Wdefestimated as an insufficient amount for decreasing the NOx to the targetdischarge amount WT; calculating the second urea feed amount Wuplas tobe consumed by the subsequent-stage SCR device 24; and feeding the ureaL of the total urea feed amount Wut, the total urea feed amount Wutbeing obtained by adding the first urea feed amount Wumol and the secondurea feed amount Wuplas.

As a result, as illustrated in FIGS. 12, 13, and 16, high NOx removalperformance can be obtained in wide ranges from low to high temperaturesin Example A of the present invention as compared to ConventionalExample B of the conventional technique. In particular, the NOx removalrate of the previous-stage SCR device 23 and the subsequent-stage SCRdevice 24 is improved by 30% or more in terms of JE05 mode average.Specifically, for the previous-stage SCR device 23, it is difficult tocontrol adsorption of urea-derived substances, and therefore increasingthe NH₃ production rate and causing a reaction of NOx and NH₃ at thesurface of the SCR catalyst allow improvement in removal rate.

Next, description will be given of an advantage for corrosion by SOx(sulfur oxides) achieved by disposing the urea injection nozzle 25upstream of the DPF device 22 as illustrated in FIGS. 1 and 3. The ureaL sprayed into the exhaust gas G from the urea injection nozzle 25produces NH₃ (ammonia) mainly through a pyrolysis reaction of the urea“(NH₂)₂CO→NH₃+HNCO” and a hydrolysis reaction of the isocyanic acidproduced by the pyrolysis reaction “HNCO+H₂O→NH₃+CO₂”. The NH₃ producedfrom the urea undergoes a reaction of “2NH₃+SO₄→(NH₄)₂SO₄” with SOx inthe exhaust gas, thereby producing (NH₄)₂SO₄ (ammonium sulfate).

Further, the (NH₄)₂SO₄ undergoes a reaction of“(NH₄)₂SO₄+CaCO₃→(NH₄)₂SO₃+CaSO₄” with CaCO₃ (calcium carbonate) whichis an ash component produced after the combustion of PMs in thedownstream (subsequent-stage) DPF device 22. The (NH₄)₂CO₃ (ammoniumcarbonate) thus produced decomposes at 58° C. or higher through apyrolysis reaction “(NH₄)₂CO₃→2NH₃+H₂O+CO₂”. The NH₃ produced by thisreaction is captured by the previous-stage SCR device 23 and thesubsequent-stage SCR device 24 downstream of the DPF device 22 and usedfor a NOx removal reaction.

The (NH₄)₂SO₄ produced by the reaction of NH₃ and SO₄ or the like is aneutralized product and thus has no corrosive properties. This solvesthe problem of corrosion of the turbine 14 and the exhaust passage 13downstream of the DPF device 22 by SOx, and also solves the problem ofcorrosion of the EGR passage 16, the EGR valve (not illustrated) and theEGR cooler (not illustrated) for the LP (low pressure)-EGR in which theexhaust gas after the reaction of NH₃ and SO₄ or the like is used as theEGR gas Ge.

Thus, according to the exhaust gas purification systems 1, 1A and theexhaust gas purification method described above, the urea injectionnozzle 25 is disposed upstream of the DPF device 22, and therefore theposition of the urea injection nozzle 25 can be closer to the enginebody 11. Thus, the temperature of the exhaust gas G to be fed with theurea L can be kept high, and therefore the rate of production of NH₃(ammonia) from the urea L can be improved.

Further, since the DPF device 22 is disposed upstream of the turbine 14,the position of the DPF device 22 is close to the exhaust ports, andtherefore the temperature of the DPF device 22 can be kept high. Thismakes it possible to increase the frequency of continuous regenerationand decrease the size. The decrease in the size of the DPF device 22 canshorten the heating time during regeneration. Thus, it is possible todecrease the amount of discharge of CO₂ during regeneration of the DPFdevice 22 and also to increase the degree of freedom in layout.

Further, since the DPF device 22 is disposed in such a way as not to beinfluenced by ash originating from the oil of the turbine 14, it ispossible to avoid the influence of the ash on clogging of the DPF device22.

In addition, in the case where the urea injection nozzle 25, the DPFdevice 22, and the turbine 14 are disposed in this order from theupstream side, SOx produced by combustion in the cylinders can bechanged to CaSO₄, which has low corrosive properties. Thus, it ispossible to suppress corrosion of the turbine by SOx.

Further, by performing the hydrocarbon feed control, the temperature ofthe exhaust gas flowing into the DPF device 22 can be raised totemperatures that allow continuous regeneration of the DPF device 22,when such continuous regeneration is being needed. Thus, the interval ofthe automatic forced regeneration control for the DPF device 22 can beextended. Accordingly, the amount of discharge of CO₂ duringregeneration of the DPF device 22 can further be decreased.

Furthermore, by the urea feed control, the urea L can be fed to the DPFdevice 22, the previous-stage SCR device 23, and the subsequent-stageSCR device 24 as a more appropriate amount of ammonia-based solution.Accordingly, it is possible to efficiently remove NOx in wide rangesfrom low to high temperatures and to high flow rates.

Thus, by combining the arrangement of the exhaust gas purificationunits, the hydrocarbon feed control, and the ammonia-based solution feedcontrol of the present invention, it is possible to improve the NOxremoval rate in wide ranges from low temperatures and rates to hightemperatures and flow rates.

INDUSTRIAL APPLICABILITY

The exhaust gas purification system and the exhaust gas purificationmethod of the present invention are capable of improving the NOx removalrate in wide ranges from low to high temperatures and to high flowrates, and also keeping the temperature of a DPF device high to increasethe time and frequency of continuous regeneration, thus decreasing thefrequency of forced regeneration of the DPF device and the amount ofdischarge carbon dioxide (CO₂) produced during the forced regeneration,by using a configuration that improves the production rate of ammonia(NH₃) and a two-stage configuration that includes a previous-stage NOxselective reduction catalyst device for high temperatures and asubsequent-stage NOx selective reduction catalyst device for lowtemperatures. Thus, the exhaust gas purification system and the exhaustgas purification method of the present invention can be utilized as anexhaust gas purification system and an exhaust gas purification methodfor internal combustion engines mounted on automobiles and the like.

EXPLANATION OF REFERENCE NUMERALS

-   1, 1A exhaust gas purification system-   10 internal combustion engine (engine)-   11 engine body-   12 exhaust manifold-   13 exhaust passage-   14 turbine of turbocharger-   15 HP-EGR passage-   16 LP-EGR passage-   21 previous-stage oxidation catalyst device (DOC)-   21 a first oxidation catalyst device (DOC-1)-   21 b second oxidation catalyst device (DOC-2)-   22 diesel particulate filter device (DPF device)-   23 previous-stage NOx selective reduction catalyst device    (previous-stage SCR device)-   24 subsequent-stage NOx selective reduction catalyst device    (subsequent-stage SCR device)-   25 urea injection nozzle (ammonia-based solution feeder)-   31 temperature sensor-   32 differential pressure sensor-   33 NOx concentration sensor-   G exhaust gas-   Ge EGR gas-   L urea-   T DPF inlet temperature-   ΔP differential pressure between upstream and downstream sides of    DPF device

1. An exhaust gas purification system for removing particulate matterand nitrogen oxides in an exhaust gas of an internal combustion engine,comprising: a previous-stage oxidation catalyst device, a dieselparticulate filter device, a turbine of a turbocharger, a previous-stageNOx selective reduction catalyst device, and a subsequent-stage NOxselective reduction catalyst device disposed in an exhaust system of theinternal combustion engine in this order from an exhaust port side, anammonia-based solution feeder disposed between the previous-stageoxidation catalyst device and the diesel particulate filter device orbetween the diesel particulate filter device and the turbine, and a NOxselective reduction catalyst of the previous-stage NOx selectivereduction catalyst device made of a catalyst containing a rare earthcomposite oxide, and a NOx selective reduction catalyst of thesubsequent-stage NOx selective reduction catalyst device made of azeolite catalyst.
 2. The exhaust gas purification system according toclaim 1, further comprising: an ammonia-based solution feed controllerfor finding, from an equivalence ratio of a chemical equation, an amountwhich enables reduction of an amount of NOx discharged from the internalcombustion engine, calculating a first ammonia-based solution amountlarger than the amount enabling the reduction, calculating a secondammonia-based solution amount from a difference between a NOx targetdischarge amount from the internal combustion engine, and an amount ofNOx measured downstream of the subsequent-stage NOx selective reductioncatalyst device, setting an amount of an ammonia-based solution to befed to the exhaust system based on the sum of the first ammonia-basedsolution amount and the second ammonia-based solution amount, andfeeding the ammonia-based solution from the ammonia-based solutionfeeder.
 3. The exhaust gas purification system according to claim 1,further comprising: a hydrocarbon feed controller for performing controlin which a hydrocarbon is fed into the exhaust gas upstream of theprevious-stage oxidation catalyst device by post injection via injectioninside a cylinder or by exhaust pipe fuel injection in a case where adifferential pressure between upstream and downstream sides of thediesel particulate filter device is equal to or higher than a continuousregeneration determination differential pressure but equal to or lowerthan an automatic forced regeneration determination differentialpressure, and a temperature of the exhaust gas at an inlet of the dieselparticulate filter device is equal to or lower than a continuousregeneration control start temperature.
 4. An exhaust gas purificationmethod for removing particulate matters and nitrogen oxides in anexhaust gas of an internal combustion engine by using an exhaust gaspurification system in which a previous-stage oxidation catalyst device,a diesel particulate filter device, a turbine of a turbocharger, aprevious-stage NOx selective reduction catalyst device, and asubsequent-stage NOx selective reduction catalyst device are disposed inan exhaust system of the internal combustion engine in this order froman exhaust port side, and an ammonia-based solution feeder is disposedbetween the previous-stage oxidation catalyst device and the dieselparticulate filter device or between the diesel particulate filterdevice and the turbine, wherein the method comprises: finding, from anequivalence ratio of a chemical equation, an amount which enablesreduction of an amount of NOx discharged from the internal combustionengine; calculating a first ammonia-based solution amount larger thanthe amount enabling the reduction; calculating a second ammonia-basedsolution amount from a difference between a NOx target discharge amountfrom the internal combustion engine, and an amount of NOx measureddownstream of the subsequent-stage NOx selective reduction catalystdevice; setting an amount of an ammonia-based solution to be fed to theexhaust system based on the sum of the first ammonia-based solutionamount and the second ammonia-based solution amount; and feeding theammonia-based solution from the ammonia-based solution feeder.
 5. Theexhaust gas purification method according to claim 4, comprising:feeding a hydrocarbon into the exhaust gas upstream of theprevious-stage oxidation catalyst device by post injection via injectioninside a cylinder or by exhaust pipe fuel injection in a case where adifferential pressure between upstream and downstream sides of thediesel particulate filter device is equal to or higher than a continuousregeneration determination differential pressure but equal to or lowerthan an automatic forced regeneration determination differentialpressure, and a temperature of the exhaust gas at an inlet of the dieselparticulate filter device is equal to or lower than a continuousregeneration control start temperature.
 6. The exhaust gas purificationsystem according to claim 2, further comprising: a hydrocarbon feedcontroller for performing control in which a hydrocarbon is fed into theexhaust gas upstream of the previous-stage oxidation catalyst device bypost injection via injection inside a cylinder or by exhaust pipe fuelinjection in a case where a differential pressure between upstream anddownstream sides of the diesel particulate filter device is equal to orhigher than a continuous regeneration determination differentialpressure but equal to or lower than an automatic forced regenerationdetermination differential pressure, and a temperature of the exhaustgas at an inlet of the diesel particulate filter device is equal to orlower than a continuous regeneration control start temperature.